1. Field of the Invention
This invention generally relates to the processing of used electrode discharged from an electrical discharge machining (EDM) system and more particularly to a wire chopper module for shearing used electrode wire into many small pieces which accumulate in a relatively dense pile.
2. Description of the Related Art
The process of electrical discharge machining (EDM) is well known. An electrical potential (voltage) is established between a continuously moving EDM electrode and an electrically conductive workpiece. The potential is raised to a level at which a discharge is created between the EDM electrode and the workpiece. The intense heat generated by the discharge will melt and/or vaporize a portion of both the workpiece and the electrode to thereby remove, in a very small increment, a piece of the workpiece. By generating a large number of such discharges, a large number of increments are removed from the workpiece whereby the workpiece can be cut very precisely to have a desired planar contour. A dielectric fluid is used to establish the necessary electrical conditions to initiate the discharge and to flush debris from the active machining area.
The EDM process erodes the wire electrode as well as the workpiece. Therefore, EDM systems continuously renew the wire electrode from a coil. Spent electrode wire has previously been allowed to accumulate in a reservoir below the EDM system. This approach proved unacceptable because the wire has a tendency for expansion in the reservoir. This necessitates frequent intervention by the operator because the wire forms entanglements, which rapidly fill the space available in the reservoir. It is known to crimp the electrode wire by making it pass between two toothed wheels or also to cut it into small pieces which possess the capability of accumulating in a relatively dense pile. A device for cutting the wire into pieces is described in U.S. Pat. No. 4,016,395.
A continuing problem with prior art wire-chopping devices is the rapid wear of the tool edges used to section the wire. EDM wire is typically comprised of tough metal allows to give it the requisite tensile strength. Chopping this wire results in high wear to cutting surfaces. If cutting surfaces wear to the extent that the wire is no longer chopped cleanly, the wire-chopping device can jam resulting in a machine shutdown.
Reliable processing of spent EDM electrode is essential to reliable operation of EDM equipment, for example, at night or over a weekend. EDM systems frequently require a large capital investment, which motivates the owner to keep the machines in use as continuously as possible. Failure of the electrode disposal device results in expensive downtime. The premium placed on reliable electrode disposal has necessitated frequent service typically including replacement of portions or the entire wire-chopping device to renew the cutting surfaces.
One prior art wire-chopping device comprises a plurality of blades mounted to a rotating shaft. A stationary wire guide orifice is positioned to feed wire into the blades generally transversely to the axis of blade rotation. The wire is sheared transverse to its feed path as the blades pass the exit orifice of the wire guide. The blades and wire guide orifice are constructed of tungsten carbide or similar hardened material to increase their durability. The blades are permanently fixed to a shaft of the wire-chopping device by welding or other known process for fixing a hard cutting edge to a cutting tool. The prior art discloses a wire-chopping device in which the wire guide orifice has two possible locations, each of which guide the spent electrode into axially spaced portions of the cutting blades. When one portion of the cutting blade is dulled, the write guide orifice is repositioned to feed wire toward a fresh cutting surface on the cutting blades. Once the cutting blades are dulled in both available locations, the prior art wire-chopping device must be removed and discarded or remanufactured by inserting a new shaft with new cutting blades.
The prior art wire-chopping devices of the rotating blade/stationary anvil type require frequent replacement to renew the permanently attached blade cutting surfaces. This results in high expense and the necessity to maintain an inventory of remanufactured and/or new wire-chopping devices. There is a need in the art for a wire-chopping device in which the cutting surfaces can be frequently and inexpensively renewed without necessitating replacement of the entire device.